for bonus task, with new branch

src/main/java/mate/academy/service/AuthenticationService.java

@@ -1,5 +1,7 @@
 package mate.academy.service;
 
+import mate.academy.model.User;
+
 public class AuthenticationService {
     /**
      * Imagine that some user wants to login to your site.
@@ -10,7 +12,16 @@ public class AuthenticationService {
      * @return true if user by email exists and passed password is equal to user's password.
      * Return false in any other cases.
      */
+    private UserService userService = new UserService();
+
     public boolean login(String email, String password) {
-        return false;
+        boolean userExist = false;
+        User user = userService.findByEmail(email);
+        if (user != null) {
+            if (user.getPassword().equals(password)) {
+                userExist = true;
+            }
+        }
+        return userExist;
     }
 }

src/main/java/mate/academy/service/UserService.java

@@ -15,6 +15,11 @@ public class UserService {
      * Return <code>null</code> if there is no suitable user
      */
     public User findByEmail(String email) {
+        for (User user : users) {
+            if (user.getEmail().equals(email)) {
+                return user;
+            }
+        }
         return null;
     }
 }